1. Field of the Invention
The invention relates in general to a digital/analog converting apparatus and digital/analog converter thereof, and more particularly to a digital/analog converting apparatus using a smaller number of conversion units and digital/analog converter thereof.
2. Description of the Related Art
Referring to FIG. 1, a structure diagram of a digital/analog converting apparatus is shown. A digital/analog converting apparatus 100 includes a binary to thermometer code unit 110, a dynamic element matching unit 120, a digital/analog converter 130 and a low-pass filter 140.
The binary to thermometer code unit 110 is for receiving and converting a K-bit digital signal Di to a N-bit digital signal Dn, wherein N is generally equal to 2^(K−1). The dynamic element matching unit 120 is for reducing the affect of the element-value deviation on the digital signal Dn and accordingly outputting the digital signal Dn to the digital/analog converter 130. The digital/analog converter 130 converts the digital signal Dn to an analog signal A0 and outputs the signal A0 to the low-pass filter 140 to generate an analog signal Al.
Referring to FIG. 2, a circuit diagram of a direct-charge-transfer digital/analog converter disclosed by Ichiro Fujimori in U.S. Pat. No. 5,990,819 is shown. A digital/analog converter 200 includes conversion units E1 to Ei, switches SW01 and SW02, a capacitor Ch0 and an operational amplifier O1.
The conversion unit E1 includes a capacitor CD1, and switches 201˜203. The switch 201 is coupled between the capacitor CD1 and the output terminal OUT. The switch 202 is coupled between the capacitor CD1 and a high voltage Vr+. The switch 203 is coupled between the capacitor CD1 and a low voltage Vr−. The conversion unit Ei includes a capacitor CDi and switches 211˜213. The switch 211 is coupled between the capacitor CDi and the output terminal OUT. The switch 212 is coupled between the capacitor CDi and the high voltage Vr+. The switch 213 is coupled between the capacitor CDi and the low voltage Vr−.
The conversion units E1 to Ei are controlled by signals φ1 and φ2. The signal φ1 is complementary to the signal φ2. The conversion unit Ei is taken as an example in the following description. When the signal φ1 is enabled, the switch SW1 is turned on, and the capacitor Ci is charged by a voltage (Vr+-Vref) or (Vr−-Vref) according to a value di. When the signal φ2 is enabled, the capacitors C1˜Ci share charges to achieve the purpose of digital-to-analog conversion and provide the voltage to the capacitor Ch after the conversion to complete first-order low-pass filtering.
However, in order that the digital/analog converter 130 can output an analog signal with N voltage levels, the digital/analog converter 130 needs to have N conversion units. When the signal OUT is required to be more accurate, the number of the conversion units has to be increased. Besides, as the number N of conversion units is increased, the circuit complication of the binary to thermometer code unit and the dynamic element matching unit will also be largely increased as well as the cost and circuit size of the digital/analog converting apparatus.